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Repeat the test after a week, and go preferably for a blood pregnancy test as it is more reliable. If this is negative then consult your gynecologist to find out why you have missed periods.
Various causes are: stress whether emotional or physical, hypothyroidism, polycystic disease of the ovary, pituitary tumors secreting prolactin, anemia and poor diet, hormonal imbalance, unexplained or sudden weight gain or weight loss. Hope this helps. Take care!